This invention relates to fuel assemblies for nuclear reactors, and more particularly to nuclear fuel rod spacers or spacer grids which are usually placed at predetermined distance(s) along the length of the fuel assembly in order to provide lateral bracing and spacing, and to maintain the nuclear fuel rods in fixed positions.
In a nuclear reactor, the reactor core contains nuclear fuel which is typically in the form of fuel rods grouped together in fuel assemblies. Groups of fuel assemblies are arranged into a matrix to form a core capable of controlled fission reaction.
Each fuel rod is typically a long member approximately 0.4 inches in diameter and 8 to 15 feet long containing fuel usually in the form of a stack of fuel pellets which are surrounded by tubular cladding. The fuel rods which make up an assembly are grouped together to form a plurality of longitudinally extending members which are supported by two parallel end plates, an upper and a lower tie plate. These plates are usually connected to one another by tie rods, or other structural elements.
Each fuel assembly or bundle may also include nonfuel bearing members. Examples include guide tubes to form passageways for control rods which assist in controlling the rate of fission, instrumentation tubes for in-core instrumentation, spacer capture rods, and water rods to modify the neutron moderation in the assembly. The spaces between adjacent fuel rods create flow channels through which coolant and/or moderator can circulate. In light water reactors, the coolant and moderator is water. Lateral bracing and spacing of the fuel rods in the fuel assembly are provided by spacers or spacer grids.
The fuel assembly or bundle, whether in a pressurized water reactor, boiling water reactor, high temperature gas cooled reactor, or any other type of reactor, functions in part to maintain the fuel rods in a fixed position, ideally free of vibration and restrained from bowing or other lateral displacement during normal and other operating conditions. In addition, by maintaining the fuel rods in fixed positions, proper cooling and neutron moderation can be achieved. Devices that assist in maintaining the fuel rods in fixed positions in the fuel assembly or bundle and which thereby facilitate proper fuel cooling are spacers.
Spacers or spacer grids which provide lateral bracing are typically designed to allow differential axial expansion of the fuel rods. Springs incorporated in the spacer grids are most frequently used to permit some sliding of the fuel rods with respect to the spacer grids. In some of the designs, the spacer grid is free to move axially a small amount to accommodate minor changes in the axial length of the fuel rods during irradiation.
If spacers were to be rigidly connected to the fuel rods as well as to structural members of the fuel assembly, then relative axial movement due to rod growth and thermal expansion of adjacent rods can cause local fuel rod skewing and bowing.
By being positioned at regular intervals, spacers maintain rod-to-rod spacing along the length of the fuel assembly. Spacers are typically made of zirconium based alloy sheet material or sometimes from Inconel or stainless steel, and are built up from a relatively large number of different intricately shaped strips that are fitted together by hand and subsequently welded or brazed. The spacers generally have an egg crate shape and each spacer cell includes features such as dimples and/or springs to maintain the desired rod-to-rod spacing. Sometimes, short sections of tubing are used that are welded to one another along parts of their edges. Thus, the springs and dimples keep the fuel rods in their proper lateral positions. But, under the influence of radiation, undesirable changes in fuel rod pitch (i.e. rod-to-rod spacing) can occur which may cause gaps or spaces to develop between fuel rods and the spacers, and increase the likelihood that the rods and/or spacer grids will vibrate. Such gaps, changes in fuel rod pitch, and vibration may lead to fuel rod fretting and failure. Furthermore, as the fuel is irradiated, the fuel rods undergo a shrinkage or diameter reduction known as "creepdown" which can result in gaps between the fuel rod cladding and the spacer which in turn can cause or contribute to fuel rod fretting.
Spacers should be thin members and have minimal cross-sectional area. Ideally, they are invisible to moderator and coolant flow while providing required lateral strength. Spacer designs reduce flow area and, also increase flow resistance and restrict coolant flow causing undesirable pressure drops. Thus, the particular physical configuration of a spacer can create or contribute to local or even non-local undesirable flow redistribution, restriction, or distortion.
Typically, the fabrication of spacers requires extensive labor in shaping the separate parts and in assembling and welding these parts to form a spacer grid. Many of these operations can be automated. However, even with automated spacer fabrication, assembly and joining, high manufacturing cost and inspection costs result from complex designs.
It would be an advantage if the effective coolant flow area surrounding each fuel rod is maximized by minimizing the spacer cross sectional surface area which impedes coolant flow.
It would be a further advantage if the assembly of the spacer was simplified by the spacer design itself.
It would be an additional further advantage if a spacer having each of the above advantages could be fabricated, tested and inspected at a lower cost than conventional spacers and at the same time improve overall quality and reliability.